1. Field of the Invention
This invention relates to a system and process for desulfurizing hydrocarbon fractions, and in particular to a system and process that integrates ionic liquid extractive desulfurization with a hydroprocessing reactor.
2. Description of Related Art
The discharge into the atmosphere of sulfur compounds during processing and end-use of the petroleum products derived from sour crude oil pose safety and environmental problems. Sulfur-containing compounds in hydrocarbon mixtures can include organosulfur compounds such as mercaptans, thiophenes, benzothiophenes, dibenzothiophenes, which can include substituted alkyl, aryl or alkaryl groups.
The stringent sulfur specifications applicable to transportation and other fuel products have impacted the refining industry and refiners will have to continue to make investments necessary to greatly reduce the sulfur content in gas oils to 10 parts per million by weight (ppmw). In industrialized countries of the United States, Japan and many countries of Europe, transportation fuel producers have already made investments and are producing environmentally clean transportation fuels. For instance, in 2007 the United States Environmental Protection Agency required sulfur content of highway diesel fuel to be reduced 97%, from 500 ppmw (low sulfur diesel) to 15 ppmw (ultra-low sulfur diesel). The European Union has enacted even more stringent standards, requiring diesel and gasoline fuels sold in 2009 to contain less than 10 ppmw of sulfur. The developing countries are following in the direction of the industrialized nations and moving forward with regulations that will require more refineries to produce low sulfur transportation fuels.
In order to keep pace with recent trends toward higher production of low sulfur fuels, refiners must choose among the processes or crude oils that provide the flexibility to ensure that future specifications are met with minimum investment by utilizing existing units and capacity. Conventional technologies such as hydrocracking and two-stage hydrotreating offer solutions to refiners for the production of clean transportation fuels. These technologies are available and can be applied as new production facilities are constructed. However, many existing hydroprocessing facilities, such as low pressure hydrotreaters, which represent substantial prior investment, were constructed before these more stringent sulfur requirements were enacted. It is very difficult to upgrade existing hydroprocessing systems because of the comparably more sever operational requirements (i.e., temperature and pressure) for clean fuel production. Available retrofitting options for refiners include increasing the hydrogen partial pressure by increasing the recycle gas quality, applying more active catalyst compositions, installing improved reactor components to enhance liquid-solid contact, increasing reactor volume and increasing the feedstock quality.
Hydrotreating and hydrocracking systems consist of two main sections: reaction and separation, the configuration of which can vary according to the particular application. In general, in systems that use either a hot separator, commonly referred to as a “hot scheme,” or in systems that use a cold separator, commonly referred to as a “cold scheme,” the effluent from a catalytic reactor is passed to a heat exchanger in which its temperature is reduced by transferring heat to the reactor feedstock. After compression, gases are recycled to the catalytic reactor and bottoms are introduced to a low pressure, low temperature separator for further separation.
There are many hydrotreating units installed worldwide producing transportation fuels containing 500-3000 ppmw sulfur. These units were designed for, and are being operated at, relatively mild conditions, e.g., low hydrogen partial pressures of 30 kilograms per square centimeter for straight run gas oils boiling in the range of 180° C. to 370° C.
However, with the stringent environmental sulfur specifications in transportation fuels mentioned above, the allowable sulfur level is being lowered to a maximum of 10 ppmw. This level of sulfur in the end product conventionally requires construction of new hydrotreating units capable of withstanding high temperature and/or pressure conditions, substantial retrofitting of existing facilities (e.g., by integrating new reactors, integrating gas purification systems, reengineering the internal configuration and components of reactors, and the like), and/or deployment of more active catalyst compositions.
Hydrocarbon mixtures can also contain nitrogen-containing compounds which often inhibit the desulfurization reactions. In a deep desulfurization process, it is therefore advantageous to also eliminate nitrogen-containing compounds. Nitrogen-containing compounds include organonitrogen compounds such as pyridines, amines, pyrroles, anilines, quinoline, and acridine, which can include substituted alkyl, aryl or alkaryl groups.
The development of non-catalytic processes to carry out the final desulfurization of petroleum distillate feedstocks has been widely studied. Prior art systems describe purification processes based on oxidation of sulfur-containing compounds, e.g., as disclosed in U.S. Pat. Nos. 5,910,440, 5,824,207, 5,753,102, 3,341,448 and 2,749,284; based on adsorption, e.g., as disclosed in U.S. Pat. Nos. 5,730,860, 3,767,563, 4,830,733; or based on the use of feedstock transfer complexes, e.g., as disclosed in PCT Patent Publication Number WO 98/56875.
A process for desulfurization of light gasoline was investigated based on precipitation of S-alkylsulfonium salts produced by the reaction of sulfur-containing compounds with alkylating agents, as reported by Y. Shiraishi et al., “A Novel Desulfurization Process for Fuel Oils Based on the Formation and Subsequent Precipitation of S-Alkylsulfonium Salts,” Ind. & Eng. Chem. Res., vol. 40, no. 22 (2001), pp. 4919-4924). While this process does not use either catalyst or hydrogen and reportedly can be operated under moderate conditions, insoluble ionic compounds are formed that must be separated, after anion metathetic exchange, by filtration.
Ionic liquids can also be suitable for desulfurizing hydrocarbon fractions by extraction. Removal rates as high as 40 W % at room temperature have been reported by X. Jiang et al., “Imidazolium-based alkylphosphate ionic liquids—A potential solvent for extractive desulfurization of fuel,” Fuel, vol. 87, no. 1 (2008), pp. 79-84, and J. Wang et al., “Desulfurization of gasoline by extraction with n-alkyl-pyridinium-based ionic liquids,” J. Fuel Chem. and Tech., vol. 35, no. 3 (2007), pp. 293-296. The processes described in the Jiang et al. and Wang et al. references use gasoline as the feedstock to demonstrate extractive desulfurization.
Non-aqueous ionic liquids of the general formula Q+A−, initially developed by electrochemists, are useful as solvents and catalysts for organic, catalytic or enzymatic reactions, as solvents for liquid-liquid separations or for the synthesis of new materials. H. Olivier-Bourbigou et al., “Ionic liquids: perspectives for organic and catalytic reactions.” Journal of Molecular Catalysis A: Chemical (2002), 182-183, 419-437. Because of their completely ionic and polar nature, these media prove to be very good solvents for ionic or polar compounds. Ionic liquids are also suitable solvents for carrying out alkylation of sulfur-containing or nitrogen-containing derivatives of sulfonium and ammonium compounds, respectively. In the Olivier-Bourbigou et al. reference, ionic liquids are used as acid catalysts for alkylation reactions.
U.S. Pat. No. 6,274,026 describes the use of ionic liquids to remove sulfur using an electrochemical process. Sulfur is removed from a stream containing hydrocarbon and polymerizable sulfur compounds by combining the hydrocarbon feed with a ionic liquid and electrochemically oxidizing the polymerizable sulfur compounds. A first fraction comprising sulfur oligomers, ionic liquid, and entrained hydrocarbon, and a second fraction comprising desulfurized hydrocarbon feed, are recovered. However, the process described in U.S. Pat. No. 6,274,026 cannot be readily integrated with existing hydrotreating facilities.
U.S. Pat. No. 7,198,712 describes a process for desulfurization and denitrification of hydrocarbon fractions. The hydrocarbon mixture is brought into contact with a non-aqueous ionic liquid of the general formula Q+A−, in which Q+ is a ammonium, phosphonium or sulfonium cation, that contains at least one alkylating agent of the formula RX−, making it possible to form ionic sulfur-containing derivatives and nitrogen-containing derivatives that have a preferred solubility in the ionic liquid. The ionic liquid is separated by decanting it from the resulting hydrocarbon mixture that is low in sulfur and nitrogen. However, such a system is described as a grass root desulfurization system, and there is no suggestion as to how such a process can be integrated in existing hydroprocessing systems.
As used herein, the term “hydroprocessing” includes hydrocracking, hydrotreating and hydrodesulfurization.
As is apparent from the above-described disclosures, ionic liquids have been proposed for use in certain types of desulfurization and/or denitrification. However, the prior art disclosures have various drawbacks. A main application of ionic liquids is to promote alkylation reactions. Other disclosures teach systems that require construction or substantial modification to existing refinery plants. Therefore, it is an object of the present invention to increase the level of desulfurization or both desulfurization and denitrification in hydroprocessing systems using ionic liquids without the drawbacks associated with prior art systems and methods.
It is another object of the present invention to provide a system and process to reduce the sulfur level, or both the sulfur and nitrogen level, of catalytic reactor effluents within the boundaries of a hydroprocessing system without requiring the addition of significant equipment or hardware to existing hydroprocessing systems.